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SHA1 (Secure Hash Algorithm 1) is a cryptographic hash function developed by the United States National Security Agency and published by the National Institute of Standards and Technology in 1995. SHA1 accepts input of arbitrary length and produces a 160-bit (20-byte) hash value known as a message digest. This digest is often represented as a 40-digit hexadecimal number.
SHA1 was originally developed as part of the larger SHA family of algorithms, and was seen as an improvement on an earlier version, SHA-0. SHA1 quickly found widespread use in various areas of information technology and cybersecurity. It was used to verify the integrity of files, digital signatures, security protocols such as TLS/SSL and SSH, and version control systems such as Git.
Over time, however, vulnerabilities in SHA1 were discovered. The key problem was its susceptibility to collisions. A collision occurs when two different input messages produce the same hash digest. Although collisions are theoretically always possible for any hash function, resistance to them is critical to security. Finding practical ways to generate collisions for SHA1 became a serious problem.
In 2017, a group of researchers demonstrated a feasible collision attack on SHA1, known as SHAttered. This meant that attackers could create two different files that generated the same SHA1 hash, allowing them to forge digital signatures or manipulate data without raising suspicion.
Due to these vulnerabilities, SHA1 has been deprecated and is not recommended for use in most applications that require a high degree of security. It is recommended to migrate to stronger hash functions such as SHA-256, SHA-384, or SHA-512, which are part of the SHA-2 family, or to newer algorithms such as SHA-3. Many organizations and software vendors have stopped supporting SHA1, and modern browsers and operating systems warn users about the insecurity of sites or applications that use SHA1.